Fingerprints are invariable, unique and convenient, which thus provide a high-level identity security authentication. During the identity security authentication, fingerprint sensors are generally used as devices for automatically acquiring the fingerprints.
Based on the sensing mechanism, fingerprint sensors are mainly categorized into optical fingerprint sensors, semiconductor capacitive sensors, semiconductor thermistor sensor, semiconductor pressure sensitive sensors, ultrasound sensors and radio frequency (RF) sensors and the like.
Using a semiconductor capacitive fingerprint sensor as an example, on a “flat panel” where thousands of semiconductor devices are integrated, another face of a capacitor is formed when a finger is attached on the “flat panel”. Since the surface of the finger is concave and convex and is not smooth, the convex points correspond to ridges and the concave points correspond to valleys, such that the actual distance from the convex points to the flat panel is different from the actual distance from the concave points to the flat panel. As such, values of the formed capacitances are different. The acquired values of the different capacitances are collected, and thus acquisition of fingerprints is complete.
However, an integration-type semiconductor capacitive fingerprint sensor generally includes an integrator. A base capacitor in a fingerprint capacitor, and a capacitor from a metal layer most proximate to the fingerprint to the system ground, that is, a parasitic capacitor may output a very great base signal at the integrator. The base signal is about 100 times of a valid signal corresponding to the fingerprint capacitor. Upon amplification by an amplifier, the base signal is far greater than the valid signal, such that the valid signal is relatively smaller. In addition, the amplified base signal may cause the integrator to simply come to saturation and cause an output dynamic range of the fingerprint sensor to become smaller.